The present invention relates to a method of joining synthetic corundum, a method of manufacturing a synthetic corundum cell using such a method of joining synthetic corundum, and a synthetic corundum cell to which such a method of manufacturing a synthetic corundum cell is applied.
Hexagonal synthetic corundum (synthetic sapphire) is excellent in terms of hardness, light transmission, and resistance to chemicals, and hence is used in a flow cell, for example, which is incorporated in a particle counter which count particles contained in a liquid such as hydrofluoric acid, for example.
Manufacturing products of synthetic corundum needs to join synthetic corundum pieces that have been cut to predetermined dimensions. However, since single crystals such as synthetic corundum pieces have different coefficients of thermal expansion dependent on the direction in the crystals, synthetic corundum pieces that have simply been joined together would tend to peel off, and are not suitable for use in flow cells through which the hydrofluoric acid or the like passes.
There has been known a method of manufacturing a structural body made of a single crystal of integral synthetic corundum as disclosed in Japanese patent publication No. 5-79640. According to the disclosed method, an ingot of a single crystal of synthetic corundum is cut into a first prism, and a surface of the first prism is optically ground to a flatness accuracy that is equal to or less than xcex/8 of the wavelength xcex (=6328 xc3x85) of red light, thus producing a second prism. Four surfaces, including the optically ground surface, of the second prism are surrounded by a jig, and cut into a first planar piece with a plane perpendicular to the optically ground surface. Then, both upper and lower surfaces of the first planar piece are optically ground to produce a second planar piece, which is cut into cut planar pieces. The cut planar pieces are separated and superposed by an assembling jig such that their upper and lower optically ground surfaces are aligned with each other for aligned crystalline planes, ridges, axes, and axial angles. A small pressure is applied to the planar pieces to completely eliminate any interference fringes on their transparent boundary surfaces to chemically pressurize and join the planar pieces into intimate contact with each other. The planar pieces are then heated at a temperature of 1200xc2x0 C. which is lower than the melting point of 2030xc2x0 C. of synthetic corundum, so that they are joined into close contact with each other.
The flatness is defined as follows: When a planar body (P) is sandwiched by two geometrically parallel planes, the flatness is expressed by the distance (f) between the two parallel planes which is minimum, and represented by a flatness X mm or X xcexcm (X is a numerical value). Therefore, the flatness accuracy that is equal to or less than xcex/8 means the flatness of at most 0.0791 xcexcm.
According to the above method of manufacturing a structural body made of a single crystal of integral synthetic corundum, a reference surface is formed on the first prism by being optically ground to a flatness accuracy that is equal to or less than xcex/8 of the wavelength xcex (=6328 xc3x85) of red light, i.e., a flatness of at most 0.0791 xcexcm. Then, a planar piece having the reference surface is cut from the first prism, and further cut into a plurality of planar pieces, which are combined together with respect to their reference surfaces so as to be superposed for aligned crystalline planes, ridges, axes, and axial angles. When a small pressure is applied to the planar pieces to completely eliminate any interference fringes on their transparent boundary surfaces, the planar pieces are chemically pressurized and joined in intimate contact with each other, not just held in intimate contact with each other. The planar pieces are then heated at a temperature of 1200xc2x0 C. which is lower than the melting point of 2030xc2x0 C. of synthetic corundum, so that they are joined into a structural body made of a single crystal of integral synthetic corundum where no boundary surfaces are present between the planar pieces and the planar pieces will not peel off.
In order to produce a synthetic corundum cell according to the above method, it is usually necessary to cut a cylindrical ingot into a prism, grind a surface of the prism to a highly small flatness of 0.0791 xcexcm, grind planar pieces cut from the prism to a highly small flatness of 0.0791 xcexcm, and manage a particular temperature of 1200xc2x0 C. Consequently, the production process is complex, inefficient as it requires very high grinding process., and needs difficult process control. The production process is not practical, and the cost of synthetic corundum cells manufactured by the method is extremely high.
The inventor has made research efforts to develop a practical technology for joining synthetic corundum through a simple process even with a certain large value of flatness, unlike the above conventional unpractical technique. The inventor attempted to superpose synthetic corundum pieces whose ground surfaces are fully held in optical contact and heat them while adjusting the heating temperature. However, this method not only produced boundaries at the joined surfaces, but also formed smears in the boundaries, resulting in a failure to meet optical requirements. After repeating trial-and-error efforts, the inventor found that a structural body which has boundaries, but contains highly reduced smears and meet optical requirements, and are practical in terms of mechanical strength, though not firm enough to be integral, can be produced by keeping only ends of joined surfaces of synthetic corundum pieces in optical contact with each other and heating them, rather than the conventional common technical approach to superpose two optical members whose entire joint surfaces are held in optical contact with each other.
The present invention has been made in view of the problems of the conventional method of manufacturing a structural body made of a single crystal of integral synthetic corundum. It is an object of the present invention to provide a method of joining synthetic corundum which allows a synthetic corundum cell having desired optical characteristics and mechanical strength to be produced efficiently in a simple manufacturing process, a method of manufacturing a synthetic corundum cell, and a synthetic corundum cell.
To solve the above problems and achieve the above object, a method of joining synthetic corundum by joining two synthetic corundum pieces according to the present invention is characterized by the steps of super-posing ground joint surfaces of two synthetic corundum pieces, holding ends thereof in intimate contact with each other, and heating the synthetic corundum pieces at a temperature equal to or lower than the melting point of synthetic corundum to join the joint surfaces of the synthetic corundum pieces. With the above method, even if the crystalline axes, ridges, and axial angles of the synthetic corundum pieces are considerably displaced relatively to each other, the synthetic corundum pieces can be joined efficiently with sufficient optical characteristics and mechanical strength. A synthetic corundum cell can thus be manufactured in a simple production cell at a reduced cost.
The flatness of said joint surfaces should preferably in the range from xc2xd to ⅙ of the wavelength xcex (=6328 xc3x85) of red light, i.e., in the range from 0.3164 xcexcm to 0.10546 xcexcm. If the flatness of the joint surfaces were greater than xcex/2 (flatness  greater than xcex/2), then no sufficient joined state may not be obtained. Demanding a flatness smaller than xcex/6 (flatness  less than xcex/6) would result in an increase in the cost and would not be practical. The temperature at which to heat the synthetic corundum pieces should preferably be in the range from 1100xc2x0 C. to 1800xc2x0 C. If the temperature were lower than 1100xc2x0 C., then no sufficient joining strength may be obtained. If the temperature were in excess of 1800xc2x0 C., then an apparatus for heating the synthetic corundum pieces would be large in size. A particularly preferable temperature range is from 1300xc2x0 C. to 1500xc2x0 C.
A method of manufacturing a synthetic corundum cell having a through hole therein according to the present invention is characterized by the steps of superposing ground joint surfaces of second and third synthetic corundum pieces on a ground joint surface of a first synthetic corundum piece, positioning the second and third synthetic corundum pieces in confronting relationship to each other with a predetermined spacing therebetween, holding ends of the joint surface of the first synthetic corundum piece and the joint surfaces of the second and third synthetic corundum pieces in intimate contact with each other, thereafter heating the synthetic corundum pieces at a temperature equal to or lower than the melting point of synthetic corundum to join the joint surface of the first synthetic corundum piece and the joint surfaces of the second and third synthetic corundum pieces to each other, producing a stacked assembly, superposing a ground joint surface of a fourth synthetic corundum piece on other ground joint surfaces of the second and third synthetic corundum pieces of the stacked assembly, holding ends of the other joint surfaces of the second and third synthetic corundum piece and the joint surface of the fourth synthetic corundum piece in intimate contact with each other, thereafter heating the synthetic corundum pieces at a temperature equal to or lower than the melting point of synthetic corundum to join the other joint surfaces of the second and third synthetic corundum pieces and the joint surface of the fourth synthetic corundum piece of the stacked assembly to each other, producing a synthetic corundum cell.
In the above method, the flatness of, and the temperature at which to heat, the synthetic corundum pieces are preferably the same as the flatness of, and the temperature at which to heat, the synthetic corundum pieces in the method of joining synthetic corundum.
A synthetic corundum cell having a plurality of synthetic corundum pieces whose joint surfaces are joined to each other with a groove or through hole defined thereby is characterized in that the joint surfaces of said plurality of synthetic corundum cells are ground and superposed, and ends of the synthetic corundum cells are held in intimate contact with each other, and the synthetic corundum pieces are joined by being heated at a temperature equal to or lower than the melting point of synthetic corundum.
In the specification, the term xe2x80x9cintimate contactxe2x80x9d represents a state in which two surfaces which are strictly analogous to each other are held in intimate contact (referred to as xe2x80x9coptical contactxe2x80x9d) with each other without the use of an adhesive, and the term xe2x80x9cJoiningxe2x80x9d represents holding the surfaces in a stronger intimate contact state (with higher peeling strength) than the optical contact state.